1. Technical Field
The present invention relates to a mobile communication terminal with a GPS function, a positioning system, an operation control method, and a program and, more particularly, to a mobile communication terminal with a GPS function which performs GPS (Global Positioning System) positioning by using assist data received from a network.
2. Description of Related art
In recent years, the A-GPS (Assisted-GPS) system that has allowed sensitive, high-speed positioning by receiving assist data from a network (see, e.g., Patent Document 1) has become dominant among GPS systems for mobile communication terminals such as a cellular phone handset. FIG. 1 is a view showing the configuration of a positioning system using the A-GPS system described in Patent Document 1. Note that, in FIG. 1, a base station 2 is connected to a communication network (not shown), and a cellular phone handset 10 is wirelessly connected to the network through the base station 2.
The cellular phone handset 10 receives and uses, as assist data, pieces of information such as an estimated time (GPS time), an estimated position of the cellular phone handset 10, the navigation data (an almanac, ephemeris, and the like) of GPS satellites 3-1 to 3-3 and uses a high-accuracy communication synchronizing clock established between the cellular phone handset 10 and the base station 2 at the time of communication with the base station 2. This realizes higher-speed, higher-accuracy positioning than the general GPS system.
It is generally desirable for GPS positioning to realize short-time, high-accuracy positioning. As shown in FIG. 2, however, there is actually a trade-off between a positioning time and an accuracy of position: a reduction in positioning time reduces the accuracy of position while an improvement in positioning accuracy requires the longer positioning time. A GPS function using the A-GPS system described in Patent Document 1 has a plurality of positioning modes (e.g., a time priority mode (A), a standard mode (B), and an accuracy priority mode (C) shown in FIG. 2) with different positioning performances. Mode setting allows positioning-time-priority GPS positioning and positioning-accuracy-priority GPS positioning.
For example, in the case of a GPS function with the feature shown in FIG. 2, predetermining the performance at a point A as that in the positioning-time-priority mode, the performance at a point C as that in the positioning-accuracy-priority mode. and the performance at a point B which is a performance intermediate between that in the positioning-time-priority mode and that in the positioning-accuracy-priority mode as the standard mode with a good positioning time/positioning accuracy balance makes it possible to perform the GPS positioning in a desired one of the three positioning modes.    Patent Document 1
Japanese Patent Laid Open Publication No. 2002-196063